In general, process filters, such as that disclosed herein, are described in W. German patent De-PS 30 22 644. The device of this patent permits various mixtures and solutions, particularly in the chemical and pharmaceutical industry, to be mixed, filtered, washed and dried. In the filter taught therein, the heating or the drying process utilizes the wall of the reaction vessel as a heating jacket. To improve the heating performance in the vessel of DE-PS 30 22 644, the container neck is double-walled to permit heating through the container neck. The increased heating surface and the additional heating effect of this vessel is limited, however, because of a relatively small neck surface.
In practice when processing temperature-sensitive products, such as is frequently the case in the pharmaceutical industry, problems of overheating can arise in which the product quality can be degraded or even spoiled. When this happens, a crust forms on the container neck which becomes a thermoinsulating layer, and the heating efficiency declines. If the crust loosens and falls into the mixture being heated, then the product mixture becomes contaminated.
In addition to the heating of the container wall, the crust may itself pose a problem. When heating the container neck and simultaneously agitating the mixture, the product mixture which is between the heated neck surface and the agitating member or paddle is not mixed. On the container neck, then, a crust forms on the heated surface, but not on the rest of the heating surface of the inner wall of the container. This leads to uneven heating as well as the formation of undesirable crust.
In addition, by heating the container neck, the addition of material to the neck walls and subsequent narrowing of the neck opening require adjustments in the various hardware used with the reaction vessel. Thus, agitating members require that greater spacings from the vessel wall be maintained so that no contacts are made with the vessel wall. Because the greater spacings must be used, the product may not be completely coated when drying and when the vessel is emptied, may not be completely discharged. The resulting loss of efficiency can be very serious, as invariably very expensive materials are involved.